1. Field of the Invention
This invention pertains to an apparatus for testing body fluid constituents. More particularly, this invention pertains to an apparatus with enhanced interstitial fluid collection.
2. Description of the Prior Art
In the prior art, there are numerous examples of apparatus for testing and determining the level of constituents in human blood. A great deal of attention has been directed to the development of techniques and apparatus for measuring blood glucose.
As noted in commonly assigned and co-pending U.S. patent application Ser. Nos. 08/321,305 and 08/136,304 (corresponding to PCT International Publication No. WO95/10223 published Apr. 20, 1995 on International Application No. PCT/US94/11580 and incorporated herein by reference), the determination of the level of a constituent of blood can be achieved by measuring the level of that constituent in other body fluids such as interstitial fluid. The aforementioned patent applications and international publication disclose a method and apparatus for a minimally invasive technique for collecting a sample of interstitial fluid through use of an extremely small needle which penetrates into the dermal layer of the skin in order to collect a low blood or blood-free sample of interstitial fluid. The collected interstitial fluid can then be analyzed for a determination of the level of constituents within the fluid. For example, the collected interstitial fluid can be analyzed for an amount of glucose with the determined amount being representative of the amount of glucose contained within the patient""s blood.
The aforementioned applications and international publication disclose the use of a ring (item 60 in FIG. 6 of the application) which surrounds the needle to create a pressure area on the patient""s skin. It is believed this leads to increase the amount of interstitial fluid being collected.
In the collection of interstitial fluid, it is desirable to increase the speed at which a sample is collected. In the absence of mechanical or other assistance, the rate at which interstitial fluid is collected through a small diameter tube or needle is very slow. Preferably, patients utilizing such equipment for home use, will be provided with a system which collects interstitial fluid at a rapid pace to ensure that a patient does not remove the needle too early in its application. Also, it is important to provide for techniques to increase a volume of interstitial fluid being collected through a needle.
When collecting any body fluid through use of a needle, it is important that the needle be a disposable item in order to prevent re-use of the needle. Such re-use can result in the transmission of disease. Where the apparatus is to be used in a patient""s home by the patient, the apparatus should be simple to use and with the needle incorporated in a disposable item. Since the needle is incorporated in a disposable item, it is important that the disposable item be amenable to low-cost manufacture. Also, in order to test the interstitial fluid, the interstitial fluid collection mechanism must be coupled with an analytic mechanism for analyzing the collected fluid. Where such a device is to be used in home by low-skilled patients, it is important that the sampler and the analytic portion of the device be mutually configured to ensure that the sampler is coupled to the apparatus in a repeatable and reliable manner to minimize errors resulting from use of the apparatus by untrained patients.
According to a preferred embodiment of the present invention, a sampler for collecting interstitial fluid from a skin layer is disclosed. The sampler includes a sampling needle having an axis and terminating at a distal end. A pressure ring surrounds the needle in spaced relation to the needle. The needle and the pressure ring are movable relative to one another along a path of travel which is generally parallel to the axis of the needle. The needle and ring are movable relative to one another between an extended position and a retracted position. In the extended position, the distal end of the needle extends beyond the ring. In the retracted position, the distal end of the needle is recessed behind the ring. A spring is provided for biasing the needle and the ring to the retracted position.